In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art various materials have been proposed for use as OSCs in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
For example, most organic amorphous semiconductors currently available are limited to mobilities in the range of 10−2 cm2/Vs. Amorphous polymers are useful for example for the large scale manufacture of organic transistors (TFTs or OFETs) for use in the backplanes of active matrix driven displays, especially flexible displays, by using solution processable techniques.
There is especially a strong need for improved p-type organic semiconductor for application in OE devices like OFETs and OPVs that can yield improved device performance. The currently available p-type OSC materials show deficiencies in light absorption, oxidative stability and charge-carrier mobility.
In particular, there is a need for improved p-type OSCs which have a high charge carrier mobility, a high solubility in organic solvents, a good processability for the device manufacture process, a high oxidative stability, a long lifetime in electronic devices, and which are easy to synthesize.
One aim of the present invention is to provide new p-type OSC materials, especially for use in OFETs and OPV devices, fulfilling the above-mentioned requirements. Another aim is to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing conjugated polymers that contain one or more phenanthrene and/or indenofluorene units but do not contain any amine-containing unit. It was also found that such conjugated polymers, when used as p-type semiconductors in OFETs, do surprisingly give better performance, in particular a higher charge carrier mobility, than analogous polymers containing amine groups.
Conjugated polymers containing phenanthrene or indenofluorene units have been disclosed for example in WO 2005/104264 A1 or WO 2004/041901 A1 for use as electroluminescent material in OLED devices. However, the preferred polymers disclosed therein are copolymers that do further comprise structural units with one or more amine groups, in particular triarylamine units. The document does neither disclose nor suggest that polymers that are free of amine-containing units are especially suitable as p-type semiconductors in OFETs or OPV devices.